1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which forms images on a recording medium, by ejecting droplets from nozzles.
2. Description of the Related Art
An inkjet type of image forming apparatus forms an image on a medium (recording medium) by ejecting ink droplets from nozzles provided in a print head. Generally, the ink used in an image forming apparatus of this kind has a large content of solvent, such as water, organic solvent, or the like, in order to reduce the viscosity of the ink.
When ink having reduced viscosity of this kind is ejected onto a permeable medium in which the ink permeates into the medium and becomes fixed inside the medium, effects of the following kind may occur, due to inadequate removal of the solvent component inside the ink. Namely, the ink may permeate and spread to a larger extent than the prescribed dot size during the permeation of the ink into the medium, the boundaries between the dots may become blurred, the spreading of the dots may be irregular rather than uniform, and the dot shape may be streaked, or the like. Spreading of the dots in this way is called “bleeding”.
Furthermore, in the case of a non-permeable medium in which the ink principally becomes fixed on the surface of the medium, unless the solvent component in the ink is removed, the ink cannot become fixed to the surface of the medium and this is an obstacle to high-speed printing.
Therefore, technology for removing the solvent from the ink deposited on the medium has been proposed (see, for example, Japanese Patent Application Publication Nos. 6-40023, 6-71873, 6-126945, 6-171076, 2000-305412, 10-157085 and 2001-179959).
Japanese Patent Application Publication No. 6-40023 discloses a transfer type inkjet printer using ink in which colored charged particles (coloring material) are dispersed in an oil-based solvent. In Japanese Patent Application Publication No. 6-40023, ions of the same polarity as the colored charged particles inside the ink are irradiated onto an ink image on a transfer medium, thereby provisionally fixing the colored charged particles forming the ink, on the surface of the transfer medium, and the solvent in the ink image on the transfer medium is then removed by means of a solvent removing device using water or an aqueous surfactant, and a concentrated ink image consisting of the colored charged particles only is transferred to the medium.
Japanese Patent Application Publication No. 6-71873 discloses technology for improving Japanese Patent Application Publication No. 6-40023. More specifically, in order to resolve the problem of image disturbances which may occur when the solid coloring component in the ink is not in a uniform charged state, a device for supplementing the charged particles of a particular polarity in the ink is provided in the ink flow path between the ink tank and the head, and solid coloring component of a polarity other than the prescribed charging polarity is removed previously, whereupon an ink image is formed on the transfer medium.
Furthermore, similarly to Japanese Patent Application Publication No. 6-40023, Japanese Patent Application Publication No. 6-126945 discloses a transfer type inkjet printer in which ions of the same polarity as colored charged particles (coloring material) in an ink are irradiated onto an ink image on a transfer medium, thereby provisionally fixing the colored charged particular constituting the ink, on the surface of the transfer medium, whereupon a metallic mesh supplied with a voltage, a polyethylene tetrafluoride mesh having a uniform pore diameter which is sufficiently smaller than the size of the colored charged particles, and a roller having a plurality of metallic pins, are used as solvent removing devices.
Japanese Patent Application Publication No. 6-171076 discloses a transfer type inkjet printer provided with a solvent recovery device which recovers the oil-based solvent from the water or the aqueous surfactant, in a solvent removing device of the transfer type inkjet printer disclosed in Japanese Patent Application Publication No. 6-40023.
Japanese Patent Application Publication No. 2000-305412 discloses an image fixing device which removes solvent before an image is fixed into the medium. According to Japanese Patent Application Publication No. 2000-305412, a voltage is applied between a solvent removing roller and an opposing roller, before entering a fixing step, causing charged colored micro-particles (coloring material) to aggregate on the medium, and the majority of the solvent which has been separated from the micro-particles is removed by the solvent removing roller.
Japanese Patent Application Publication No. 10-157085 discloses a recording apparatus which ejects droplets of oil-based ink onto an intermediate transfer body having a surface made of a silicon member. According to Japanese Patent Application Publication No. 10-157085, when the ink adheres to the silicon member, the viscosity of the ink is increased at the boundary between the silicon member and the ink, the ink in the vicinity of the boundary solidifies completely, and the surface of the adhering ink layer assumes a state of increased viscosity. Thereupon, the ink is transferred in this state to the medium, and printing results having little bleeding are obtained.
Japanese Patent Application Publication No. 2001-179959 discloses an image forming apparatus in which a solvent absorbing body having a surface which has good separating characteristics with respect to the dye or pigment-based coloring agent (coloring material) in the ink is placed in contact with the ink on the medium, and absorbs the solvent in the ink. According to Japanese Patent Application Publication No. 2001-179959, when using a dye-based ink, the coloring agent and the solvent are separated by means of an aggregation promoter which causes the dye to aggregate and separate.
However, the technology disclosed in Japanese Patent Application Publication Nos. 6-40023, 6-71873, 6-126945, 6-171076 and 10-157085 can only be applied to transfer type recording apparatuses which use inks having an oil-based solvent, and they cannot be adapted to recording apparatuses which eject ink droplets directly onto the medium.
Japanese Patent Application Publication No. 2000-305412 simultaneously applies a voltage to a solvent removal roller and absorbs solvent on the medium by means of a solvent absorbing roller, and hence there is a risk that the coloring material will be absorbed into the solvent removing roller together with the solvent, before the coloring material aggregate on the medium.
Japanese Patent Application Publication No. 2001-179959 increases the separating characteristics between a coloring material and a solvent absorbing body, but there is still a risk that coloring material will adhere to the solvent absorbing body, together with the solvent.
In this way, as yet, there has been no proposal for an image forming apparatus which is able to remove solvent swiftly and reliably from ink deposited on a medium.